1. Field of the Invention
The present invention relates to an optical tomographic image photographing apparatus arranged to photograph a tomographic image of an examined object.
2. Description of Related Art
Conventionally, as an optical tomographic image photographing apparatus arranged to photograph a tomographic image of an examined object, there is known an optical coherence tomograph (OCT) using low coherent light (see Japanese Patent Application Unexamined Publication No. 2006-212153). This apparatus is used for obtaining a tomographic image of a living body part such as an eyeball and a skin, for example.
In order for such an apparatus to obtain a tomographic image which is formed by interference light of measurement light projected onto the examined object and reference light, it is necessary to perform adjustment of the optical path length by moving an optical-path-length varying optical member (e.g., a reference mirror).
For example, in order for an apparatus arranged to photograph a tomographic image of a fundus of an examinee's eye to obtain a fundus tomographic image which is formed by interference light of measurement light projected onto the fundus and reference light, it is necessary to perform adjustment of the optical path length by moving an optical-path-length varying optical member (e.g., a reference mirror) corresponding to an eye axial length of the examinee's eye.
In a conventional technique of performing the above-described adjustment of the optical path length automatically, the reference mirror is moved from an initial position at which the optical path length of the reference light is shortest in a direction such that the optical path length is made longer, the presence or absence of the fundus tomographic image is determined at each moving position, and the reference mirror is moved based on a result of the determination.
When the fundus tomographic image is obtained by performing Fourier analysis on a detection signal obtained by photo-receiving the interference light, a normal image or an inverted image of the fundus tomographic image is obtained in accordance with the position of the reference mirror (sometimes no fundus tomographic image is obtained). In the technique described above, the reference mirror is moved from a position at which the optical path length of the reference light is shortest or a position at which the optical path length of the reference light is longest, by which a firstly obtained fundus tomographic image is previously set to be a normal image or an inverted image. For example, in the case of moving the reference mirror from the position at which the optical path length of the reference light is longest, the firstly obtained tomographic image is set to be the inverted image. Therefore, at the time when the presence of the fundus tomographic image is detected, the firstly obtained tomographic image is identified as the inverted image of the fundus tomographic image, and the reference mirror is moved from the position at which the inverted image is obtained in a direction such that the normal image is obtained, so that the normal image is displayed on a screen of a display monitor.
However, the technique described above requires long time to perform the adjustment of the optical path length because the optical-path-length varying optical member should be moved from the position at which the optical path length of the reference light is shortest or from the position at which the optical path length of the reference light is longest. In addition, a tomographic image desired by an examiner could not be displayed if the examinee's eye blinks when the optical-path-length varying optical member is located at a position at which the previously specified tomographic image (e.g., an inverted image) is obtained during the adjustment of the optical path length.